The compound oxide is an oxide in the form of a compound of two or more kinds of metal oxides, and has no ion of oxo-acid as the unit of a constitution. One important application of the compound oxide is a catalyst and a catalyst carrier. As the catalyst of this kind, there is known a catalyst having such a constitution that catalytically active fine particles are carried on the heat-resisting carrier and used for purifying the exhaust gas of the internal combustion engine.
A variety of methods are known for producing the aforementioned catalytically active fine particles or carriers, and the method for carrying the fine particles on the carrier is described in Japanese Patent Publication No. 08-17942 or Japanese Patent Laid-Open No. 2000-197822.
The aforementioned method described in Japanese Patent Publication No. 08-17942 can be called the water-absorption carriage method. This method comprises: preparing heat-resisting powder carrying a precious metal, a perovskite type compound oxide and a heat-resisting carrier; preparing slurry containing the heat-resisting powder carrying the precious metal and the perovskite type compound oxide; applying the slurry to the heat-resisting carrier; and then drying and sintering the slurry.
On the other hand, the method described in Japanese Patent Laid-Open No. 2000-197822 can be called the impregnation carriage method. This method comprises: adding carrier powder to a solution of salt containing a metal composing a compound oxide; evaporating and drying the solution into a solid; and then sintering the solid.
In either of the methods, however, water-soluble salt such as water-soluble nitrate is dissolved in water and absorbed by the carrier already prepared, and is then dried and sintered. In this process, the carrier may be partially dissolved to have its heat resistance lowered, and the compound oxide obtained may be thermally deteriorated.